Electrode composition for electron-discharge devices



Feb. 11, 1930. s. DUSHMAN 1,747,064

ELECTRODE COMPOSITION FOR ELECTRON DISCHARGE DEVICES Filed July 2, 1925 MAG/VES/UM 0R CALCIUM Inventor: Saul Dushmafi,

His Attorney.

Patented Feb. 11, 1930 UNITED STATES PATENT OFFICE SAUL DUBHIAN, OI SCHENECTADY, NEW YORK, ASSIGNOR '10 GENEBIL ELECTRIC COMPANY, A CORPORATION OF NEW YORK v ELECTRODE COMPOSITION FOR ELECTRON-DISCHARGE DEVIOEB Application filed July 2,

The present invention relates to the constitution of cathodes for electrical discharge devices, as for example, radio detectors, rectifiers and the like.

It is the object'of my invention to provide a cathode material capable of high electron emission at an operatin temperature which is relatively low and w ich, therefore, permits of operation of the device with a low heating current capable of being furnished by an ordinary primary battery.

I have discovered that the element cerium is capable of a high electron emission at a temperature as low as 1000 to 1250 C. and I have found that when ceria, an oxide of cerium, is incorporated into an electrode which isoperated under conditions permittin reduction of at least part of the ceria, w ereby the electron emissivity of the electrode is very materially improved.

When making an electrode embodying my invention, a finely divided cerium compound is incorporated with the material constituting the main body of the electrode, as distinguished from a surface application. For example, in the manufacture of a tungsten cathode embodying my invention, a suitable nonvolatile compound of cerium is mixed with finely divided metallic tungsten from which cathode filaments are fashioned either in accordance with the Coolid e mechanical working process described in S. Patent 1,082 933 of December 30, 1913, or by squirting the nely divided material admixed with a suitable binder.

When manufacturing drawntungsten cathodes about one to two per cent of ceria (eerie um dioxide), is mixed with the finely divided tungsten metal before it is pressed into slugs preparatory to sintering. In general, as much ceria is added as may be tolerated in the tungsten Without interfering with the subsequent swaging and drawing in accordance with Coolidge Patent 1,082,933.

In some cases it may be more convenient to form filaments from a mixture of metal and ceria or other suitable compound of cerium by adding a suitable binder in sufiicient guantity to produce a plastic mass, and saplrtmg the same into filaments or ribbons. or ex- 1923. Serial No. 848,878.

ample, filaments may be made by squirting a mlxture comprising by weight 95 parts tungsten, 5 parts ceria, and enou h carbonaceous binder to form a mass capable of being squirted, thereupon baking, treating to remove the binder and sintering in the usual way.

When filaments made as described are mounted as cathodes in an electron discharge device, for example, such as shown in the drawing, comprising a bulb 1, a cathode 2, an anode 3 and a grid 4, precautions should be taken to thoroughly evacuate the bulb and contained parts, especially to remove water vapor. The electron device is provided as usual with the cathode leads 5, 6, an anode lead 7 and a grid lead 8, all of which are sealed into a stem 9 and connected to terminals (one numbered 10 being shown) sealed into a base 11. However, as these structural features form no essential part of my invention they will not be described in greater detail. Preferably, but not necessarily, a material of low vapor pressure which is highly reactive to water vapor, as for example, magnesium or calcium, is introduced into the bulb. In the device shown it is present as a film 12 on the bulb wall. For illustrative purposes a radio detector has been shown in the drawing as one of the useful devices in which my improved electrode ma be utilized, but it may be used in other evices of greater power.

When the evacuation is complete, and preferably when the bulb has been sealed, the cathode is activated b heating to a temperature of 2500 C. for about a minute and then formed at 1200 to 1400 C. for a few minutes. Heating to 2500 C. results in a reduction of a small part of the seria to metallic cerium, which difi'uses at the forming temperature to the cathode surface where it remains in greater part as an adsorbed film. The temperature of normal operation preferably is materially lower, usually below about 1200 C. as at higher temperatures cerium tends to be vaporized from the incandescent tungsten. At 1600 C. and higher the evaporation is uite rapid. In general, it maybe said that t e cetiated filaments should be operated at a bright oran e heat below white incandescence. Gatho es containing ceria after having been activated and formed as above described, when operated at 1200 0., give an electron emission of about 10 milliamperes per sq. cm. of emitting surface, whereas pure tungsten at the same temperature has the extremely low electron emission of .00004 of a milliampere per sq. cm.

Conveniently, but not necessarily, a reducing agent, for example, carbon is incorporated with the ceria into the cathode in order to assist in reducing the ceria, particularly when for any reason water vapor is evolved even though in minute quantity during the aperation of the device. Carbon may be introduced by heating the ceriatedfilament in the pres ence of the vapor of a carbon compound, as for example, the vapor of anthracene or naphthalene. The pressure of this vapor should be maintained at a low value and the temperature of the cathode, the time of treatment and other conditions should be so regulated as to maintain the carbon content less than about one per cent and preferably as low as about one-half per cent.

Although the manufacture of a tungsten base electrode has been described for the purpose of illustrating my invention, I Wish it to be understood that the benefits of my invention also may be obtained in the case of electrodes consisting wholly or largely of other metals, as for example, molybdenum or nickel.

What I claim as new and desire to secure by Letters Patent of the United States, is

1. The process of treating an electrode containing cerium as a constituent which consists in heating said electrode in non-oxidizing environment to about 2500 (1., then heating to about 1200 to 1400 C. and finally operating said electrode at a lower temperature.

2. The process of treating an electrode containing ceria in a non-oxidizing environment which consists in heating said electrode to about 2500 C. for about one minute, then heating to about 1200 to 1400 C. for several minutes andfinally operating said electrode at a temperature below and adjacent to 1200 C.

3. The process of treating a tungsten electrode containing ceria for use in an electrondischarge device which consists in heating said electrode to about 2500 C. for about one minute, then heating to about 1200 to 1400 C. for several minutes, and finallyoperating said electrode below about 1200 C., all of said operations being carried out in a vacuum free from traces of Water va or.

In witness whereof, I ave hereunto set my hand this 30th day of June, 1923.

SAUL DUSHMAN. 

